The present invention relates to semiconductor-on-insulator integrated circuit structures and fabrication methods.
Background
Semiconductor on Insulator (SOI) material is being used for digital applications, where it is able to produce material that has significantly improved performance over the conventional (bulk) material. SOI has two disadvantages when attempts are made to do analog design. These disadvantages are impaired thermal dissipation due to the oxide under the active components, and the fact that the transistor body is typically left floating.
Power is dissipated in the MOS channel as a result of the current through the channel and the voltage across it. Due to the poor thermal conductivity of SOI compared to bulk, the temperature of the layer above the buried oxide may increase significantly just in the power-dissipating device. This affects the characteristics of the device. The most typical effects of self-heating within a device are for the device to exhibit reduced threshold voltage and drive current, and to show increased leakage.
Self-heating effects are of highest significance in devices where continuous current flows through a device that is biased with the source and drain at a significant voltage separation (e.g., Vds=1v). This occurs most often in analog designs. One of the simplest analog circuits in which this is important is the current mirror.
For analog circuitry where there is a matching requirement, the floating body can be an issue because of its history effect, which can leave adjacent devices with different body voltages. Body voltage variation directly causes Vt variation. Body voltage is the voltage on the parasitic base of the SOI transistor. As body voltage increases, the threshold voltage of the device decreases due to body biasing effects.
Typically, body ties are used to alleviate the problems of a floating body. The layout is modified so that a body connection can be brought out from the device. Though this method can solve some of the problems, it does not address thermal mismatch, and takes additional silicon area to bring out the body tie.
Using SOI, the oxide layer alleviates the need for a body tie. However, the insulator used in SOI devices can have as much as 100 times the thermal resistance as the semiconductor material, which causes the devices to heat up when turned on.
Misalignments of poly on the gates of coupled devices can cause process variations between them as well. Though a slight misalignment of the poly will not affect threshold voltage, it will affect the drive currents, because of the difference between the source and drain widths caused by the misalignment.
Thermal Coupling of Matched SOI Device Bodies
The present inventor has realized that SOI implementation of analog circuits presents a possible problem of thermal differences between devices which are supposed to be matched. This problem is caused by the great difference in thermal conductivity between the insulating and semiconductor materials typically used for SOI. This problem can be solved by ensuring that the bodies of supposedly matched devices are thermally coupled by at least some continuous route of non-insulating material. In the preferred embodiment this material is a semiconductor material.
In another embodiment, coupling is instead achieved by use of metal interconnects to thermally couple different devices. This variation has the advantage of thermally coupling devices in different silicon tanks, though the coupling is typically of lower quality.
Advantages of the disclosed methods and structures, in various embodiments, can include one or more of the following:    good thermal and electrical coupling between linked devices;    thermal and electrical isolation from external sources